Tilt-up building construction generally consists of concrete wall panels that are precast horizontally on the ground, cured, and then tilted up into place. The roof framing systems of these buildings generally consist of trusses, girders, beams, purlins, joists and stiffeners that can be incorporated into wall tie and diaphragm continuity systems.
In areas subject to high seismicity, the connection between the walls of older tilt-up buildings and their timber roof framing system is generally inadequate per the currently established seismic design standards. Accordingly, seismic upgrading of such structures is occurring in older tilt-up buildings in order to mitigate these inadequacies. Such upgrades typically consist of the retrofit installation of wall tie systems having many individual components attached to the concrete wall panels and the roof framing systems.
For example, a flare strut system, as explained in U.S. Pat. No. 6,493,998, can be used to transfer forces between a concrete wall panel and a roof diaphragm continuity element in a building's roof framing system as part of a seismic upgrade project. The flare strut system comprises a plurality of elongated strut elements that connect between the wall panel and a diaphragm continuity element. For the wall panel, an end connector assembly is attached to the wall. The end connector assembly is typically attached to the wall panel by drilling holes through the wall panel and passing a threaded bolt through an anchor plate located on the exterior side of the wall panel, the hole drilled through the wall panel, and the base plate of the end connector. A nut is then threaded onto the bolt from the interior of the building to secure the anchor plate, bolt and end connector to the building. The use of an exterior anchor plate may detract from the aesthetics of the building or may be very costly to install due to the removal and replacement work of the building's exterior finish. Furthermore, the installation of an end connector assembly with an exterior anchor plate may be impossible in certain situations (e.g., the presence of an adjacent building).
Alternatively, the end connector assembly can be attached to a concrete wall using an epoxy anchor if the wall has sufficient thickness. Specifically, a 1 in. diameter threaded rod is installed in a corresponding hole drilled in the concrete wall panel. The threaded rod is secured to the wall panel with epoxy adhesive and the end connector assembly is attached to the threaded rod with a nut. With this type of installation, an exterior plate anchor is not needed.
In order to provide the necessary strength for the flare strut system using an epoxy anchor, the threaded rod must be embedded into the concrete wall panel at least 5.75 inches, and typically 6.75 inches. Generally, these embedment depths will require that the thickness of the concrete wall panel be at least 7 to 8 inches thick. Because most concrete wall panels are typically 6 inches thick, the maximum anchor embedment depth is generally limited to 4.5 to 5 inches. Accordingly, because of the decreased anchor embedment depth, the capacity of an epoxy anchor is generally found to be insufficient.
Another problem with epoxy anchors is that when large diameter epoxy anchors are installed in shallow embedments, it is possible the concrete will fail before the anchor, and thus precipitate a brittle failure mode Brittle failure modes are undesirable and should be avoided whenever possible. Brittle epoxy anchor failure modes are avoided by providing a sufficient anchor embedment depth so as to precipitate a ductile failure of the threaded rod element of an epoxy anchor before the concrete experiences brittle failure. However, as previously explained above, it may not be possible to provide a sufficient embedment depth for the epoxy anchor due to the thickness limitations associated with tilt-up concrete wall panels.